A manual transmission of a hybrid drive of the initially named type has a force transmission branch of an internal combustion engine and a force transmission branch of an electric motor that are combined at the output shaft. The force transmission branch of the internal combustion engine comprises an input shaft, the gear wheel sets of the first associated group and the output shaft, and allows the transmission of torque between the internal combustion engine and the drive wheels of the motor vehicle in drive connection with the output shaft. The force transmission branch of the electric motor comprises the other input shaft, the gear wheel sets of the second associated group and the output shaft, and allows the transmission of torque between the electric machine and the drive wheels of the motor vehicle. Thus, a driving operation based solely on the internal combustion engine is possible via a shifted gear wheel set of the first associated group, a driving operation based solely on the electric motor is possible via a shifted gear wheel set of the second assigned group, and a combined driving operation of both aggregates (internal combustion engine and electric machine), with a motor or generator operation of the electric machine is possible in each case via a shifted gearwheel set of both groups. Additionally, gear changes within a group of gear wheel sets can be performed as shifts under load, in that during the gear change the tractive force is at least partially maintained by the aggregate (internal combustion engine or electric machine) associated with the other group of gear wheel sets, via a gearwheel set engaged there.
The document DE 199 60 621 B4 describes one such hybrid drive having three embodiments of a specific manual transmission. In all embodiments, the internal combustion engine force transmission branch of the manual transmission has a first countershaft, which, on the input side, can be brought in drive connection with the drive shaft of the internal combustion engine via an input constant, a first input shaft and a friction clutch, and, on the output side, can be brought in drive connection with the output shaft via a first group of selectively shiftable gear wheel sets. In the first embodiment according to FIG. 1 contained in the document, a second input shaft is designed as a second countershaft, which is connected in a rotationally fixed manner to the rotor of an electric machine, and via a second group of selectively shiftable gear wheel sets, can be brought in drive connection with the output shaft. The two input shafts can be coupled together by a clutch disposed between the countershaft-side drive gear of a drive step disposed between the first input shaft and the second countershaft, and the second countershaft. In the second and third embodiments according to the FIGS. 2 and 3 contained in the document, the second input shaft is implemented as a hollow rotor shaft, which is disposed coaxially over the first input shaft and is connected in a rotationally fixed manner to the rotor of an electric machine disposed coaxially over the first input shaft. The rotor shaft is in drive connection, via a second input constant, with the second countershaft, which can be brought in drive connection with the output shaft, via the second group of selectively shiftable gear wheel sets. The two input shafts can be coupled together by a clutch disposed between the first input shaft and the rotor shaft.
With a further such hybrid drive, which is disclosed in the document WO 2008/138387 A1, the two input shafts of the respective manual transmission are designed as countershafts. The first input shaft, or countershaft, of the internal combustion engine force transfer branch can be connected on the input side to the drive shaft of the internal combustion engine, via a controllable separating clutch, and on the output side can be brought in drive connection with the output shaft, via a first group of selectively shiftable gear wheel sets. The second input shaft, or countershaft, of the electric motor force transfer branch can be connected on the input side in a rotationally fixed manner to the rotor of the electric machine, and on the output side can be brought in drive connection with the output shaft via a second group of selectively shiftable gear wheel sets. The gear wheel sets of both groups, by using in each case a common output gear disposed on the output shaft, are disposed in a common radial plane, which results in a relatively compact axial constructive length. The idler gears and the associated gear clutches of two axially adjacent gear wheel sets of the second group, combined in a shift packet, are disposed on the output shaft. As a result, the two counter shafts can be coupled together, via one of the respective gear clutches disposed on the counter shaft, independently of the shifting of one of the respective gear sets.
A disadvantage of the two named hybrid transmissions is however that the respective manual transmissions are special designs that are accordingly expensive to manufacture.
In contrast, several designs of such a hybrid drive are known from the unpublished document DE 10 2010 030 569 A1, with which the respective manual transmission can be derived from a double clutch transmission having two coaxially disposed input shafts and a common output shaft, and as a result can be manufactured economically. Whereas the centrally disposed first input shaft can be connected, via a separating clutch, to the drive shaft of the internal combustion engine, the second input shaft, designed as a hollow shaft, and disposed coaxially over the first input shaft, is in drive connection directly, or via an input transmission step, with the rotor of the electric machine. The shiftable gear wheel sets are each disposed between one of two countershafts in drive connection with one of the input shafts, in each case via an input constant, and the output shaft, wherein in each case gear wheel sets assigned to two different countershafts are disposed in a common radial plane with an arrangement of the idler gears on the respective counter shaft and by using a common fixed gear disposed on the output shaft. A coupling shift element, disposed externally to the transmission, directly between the two input shafts, for coupling the two input shafts, is designed as a friction clutch or as a claw clutch.
However, a disadvantage of the last named hybrid drive is that the axial constructive length of the manual transmission is relatively large due to the axial staggering of the gear wheel sets and the input constants. This inevitably leads to axially large dimensions of the entire hybrid drive which makes the use thereof at least difficult in a motor vehicle, particularly with a front-transverse arrangement.